Particles of siliceous solids have found wide utility in many fields. Among other uses, they have been used to fill, extend, thicken and reinforce various organic materials such as rubber and other elastomers. Since the usefulness of these siliceous solids has been hampered and retarded by their surface characteristics, i.e., strong affinity for water and a relatively low affinity for organic materials, these materials have been treated to increase their affinity for organic materials and decrease their affinity for water. These surface treatments have included the introduction of various substances to render the siliceous materials organophilic. Among the various materials used to treat these siliceous materials to render them organophilic have been primary and secondary alcohols, as taught by Iler in U.S. Pat. No. 2,727,876; and partially hydrolyzed esters of hydrocarbon substituted silicas, as taught by Vanderbilt et al. in U.S. Pat. No. 3,350,345. This treatment of siliceous materials to make them organophilic requires the use of expensive chemicals, elevated temperatures, and special handling procedures, all of which result in economic penalties as regards the finished product. Furthermore, it has generally been accepted in the prior art that plastics filled with organophilic minerals have better physical properties than those filled with minerals which are not organophilic. It is generally believed that the organic molecule which is chemically attached to the mineral surface forms a bond to the plastic or becomes entangled in the polymer chain, thus resulting in a stronger product.
It has now been discovered that washing siliceous materials, especially talc and kaolinite, with an aqueous solution of an acid and incorporating these acid treated minerals into polyvinyl chloride results in an improved polyvinyl chloride composition having increased tensile strength.
The acid washing does not make these minerals organophilic and there is no need with the acid treatment of the present invention to make these minerals organophilic before they are blended with the polyvinyl chloride compositions.